The present invention relates to carbon nanofibers which exhibit improved wettability with a matrix material, a method of producing the carbon nanofibers, a carbon fiber composite material using the carbon nanofibers, and a method of producing the carbon fiber composite material.
Carbon nanofibers have been known as a filler which is dispersed in a matrix to only a small extent. The inventors of the invention proposed a method of producing a carbon fiber composite material which improves the dispersibility of carbon nanofibers so that the carbon nanofibers can be uniformly dispersed in an elastomer (see JP-A-2005-97525, for example). According to this method of producing a carbon fiber composite material, the elastomer and the carbon nanofibers are mixed so that the dispersibility of the carbon nanofibers with strong aggregating properties is improved due to a shear force. Specifically, when mixing the elastomer and the carbon nanofibers, the viscous elastomer enters the space between the carbon nanofibers while specific portions of the elastomer are bonded to highly active sites of the carbon nanofibers through chemical interaction. When a high shear force is applied to the mixture of the carbon nanofibers and the elastomer having an appropriately long molecular length and a high molecular mobility (exhibiting elasticity), the carbon nanofibers move along with the deformation of the elastomer. The aggregated carbon nanofibers are separated by the restoring force of the elastomer due to its elasticity, and become dispersed in the elastomer. Expensive carbon nanofibers can be efficiently used as a filler for a composite material by thus improving the dispersibility of the carbon nanofibers in the matrix.
Carbon nanofibers are industrially mass-produced by a vapor growth method that pyrolyzes a gas such as a hydrocarbon gas in the presence of a metal catalyst. Carbon nanofibers which are mass-produced by the vapor growth method in a heating furnace at about 1000° C. and further graphitized by heating at a higher temperature have been known (see JP-A-2006-198393, for example). JP-A-2006-198393 discloses that the heating temperature for graphitizing the carbon nanofibers is 2000° C. or more, preferably 2500° C. or more, and particularly preferably 2800 to 3200° C. or more. The carbon nanofibers thus graphitized have a surface with a small number of defects. However, such carbon nanofibers exhibit poor wettability with a matrix material (e.g., elastomer).